The invention relates to a method for manufacturing cholesteric reflective polarizer that can be used in LCD devices.
The overall light efficiency of a typical LCD is so low only around 4%-6% of the brightness provided by the back light module unit. Every of constructing components of an LCD more or less will block part of light from back light module unit, such as the dichroic polarizer, color filters, or pixel aperture. LCDs rely on polarized light, however the dichroic transmittance of a regular polarizer is only around 40% and the rest part of light is absorbed by the polarizer itself. Therefore, a brightness enhancing-method by using cholesteric reflective polarizer is proposed. It is well-known that cholesteric liquid crystals are characterized with its chirality and molecular helical structure. It can selectively reflect certain circularly polarized light but not absorbing it. This is obviously different from regular dichroic polarizers. As known, cholesteric liquid crystal will reflect the circular polarized light of the same handedness to its helix, and let the circularly polarized light of opposite handedness pass through the film. By means of a simple reflection surface, the reflected light from the cholesteric liquid crystal film can be easily changed its handedness and passed through the film again. Therefore, the originally reflected light is redirected back to the film, resulted in theoretically a double intensity light with the same circular polarization. The light efficiency is then doubly increased. Furthermore, through a xc2xc-wavelength retardation film, the circularly polarization can be transformed into linearly polarization which can then be used in LCDs.
Typically, cholesteric reflective polarizer can be fabricated by xe2x80x9cone-layerxe2x80x9d method or by xe2x80x9cmulti-layerxe2x80x9d method. The former deals with a whole visible light region via a single cholesteric layer. That is, there should be an adequate variation of the helical pitch of the cholesteric liquid crystal in the thickness direction within the layer. As disclosed in U.S. Pat. Nos. 5,506,704, 5,691,789, 6,099,758, 6,057,008, and 6,071,438, the variation of the helical pitch should be achieved via a long-term low-energy exposure. Thereby, the helical pitch in the thickness direction is varied progressively. However, a long-time exposure should be problematic in mass production. Although U.S. Pat. Nos. 5,948,831 or 6,193,937 discloses the use of a laser for detecting the reaction stop point, but the time required is still substantially long and therefore is difficult to implement. Furthermore, in order to produce a substantial variation of the helical pitch within a typically 20xcx9c30 xcexcm-thick film, an exposure system that can provide ultra-low energy is required. Unfortunately, exposure systems commercially available have a power that is excessively high with respect to the entire structure. Therefore, the disclosure of the above-mentioned patents further calls for the necessary use of a UV-region neutral-density filter to reduce the power to {fraction (1/30)}. However, this neutral-density filter is excessively expensive and, furthermore, a long-term exposure operation under the above high-powered system can be problematic to the shelf life.
Therefore, considering the convenience of mass production, the multi-layer method seems to be a better solution. As disclosed in U.S. Pat. No. 6,016,177, a method for producing a circularly polarized separated plate which comprises coating an oriented cholesteric liquid crystal polymer layer with a different kind of cholesteric liquid crystal polymer and heat orienting the resulting coated layer, to provide multistage changes in helical pitch in the direction of thickness. As disclosed in the above-mentioned patent, the pre-polymerized polymer solution was first coating on a plastic substrate which has been already aligned treatment. After the solvent is evaporated, the orientation of cholesteric liquid crystal is achieved by thermal annealing for a while where the annealing temperature is within a range between the glass transition temperature Tg and the isotropic phase transition temperature. Thereafter, the temperature is decreased to under Tg to freeze the orientation structure. This method has several constraints. For example, the glass transition temperatures of the polymers must be higher than the ambient temperature. As disclosed in U.S. Pat. No. 6,016,177, with respect to a 2xcx9c6 layers lamination, the further conditions are not considered similar: a next layer of coating is directly coated on a previous already oriented layer of polymers, and the new orientation conditions would interfere to the already obtained orientation result. Moreover, the temperature can easily affect the orientation effect. This method therefore needs to be improved. Furthermore, the lamination process, used to stack several layers and to fix the orientation of the layers, usually requires the use of laminating rollers. The temperature of the laminating roller also plays an important role on the orientation of the film. Furthermore, as mentioned, the high-temperature annealing is performed around 130xc2x0 C. to 160xc2x0 C. for 3xcx9c20 min of operation, which may be problematic for the choice of the plastic substrates.
It is well known that the degree of orientation affects the performance of the brightness enhancement film. As disclosed in U.S. Pat. No. 5,599,412, a known method of producing aligned cholesteric liquid crystal inks. It firstly submits the cholesteric liquid crystal polymer to a hot molten state. Thereafter, the cholesteric liquid crystal polymer is directly coated on a conveyor belt (without alignment layer). Then, a perfect orientation is achieved via an electric field or a magnetic field. However, the maintenance of a steady electric/magnetic field is practically not convenient.
It is therefore a principal object of the invention to provide a spontaneous alignment method for manufacturing cholesteric reflective polarizer that can increase the brightness of a LCD device without the need of a specific alignment layer or mechanic rubbing to generate an adequate orientation of the cholesteric liquid crystal.
It is another object of the invention to provide a spontaneous alignment method for manufacturing cholesteric reflective polarizer in which photopolymerizable monomers or oligomers are used as starting substances that are formulated with photo-initiator and solvents directly used in the coating process, so that no preliminary polymerization is needed.
To accomplish the above and other objectives, the method of the invention first provides a first substrate. A photopolymerizable liquid crystal is formed on the first substrate. A second substrate then is formed on the liquid crystal substance to form a sandwich structure. The sandwich structure is subsequently submitted to a lamination process that generates shear stress thereon. Finally, an energetic irradiation including UV irradiation is performed to solidify the layer of liquid crystal into a liquid crystal polymer film. The second substrate then is selectively removed. The above steps are repeated until a desired reflected wavelength range is obtained.
To provide a further understanding of the invention, the following detailed description illustrates embodiments and examples of the invention, this detailed description being provided only for illustration of the invention.